Related bugs

Related FAQ:

Ubuntu does not "require" any package. The package management system only takes care that all packages are installed, if they are a dependency of another package. This leads to several tree-like dependency chains, each of them going up to a package that is marked as "manually installed". In case that you uninstall one of the "manually installed" packages, all packages in the chain below that package will be shown as "no longer required" (unless needed by another manually installed package, or manually installed by themselves).

As far as I know, intel-microcode and amd64-microcode are dependencies of the linux-image-generic package which in turn is a dependency of the package linux-generic. If you do not have one of these package installed, then the package management system does not see a need for the microcode packages and rates them "no longer required".

I suggest that you check whether you have the package linux-generic (or eventually linux-image-generic) installed, and if not (which I assume), then you should install it to make sure that the microcode packages stay installed.

(Yes, I know, this will install the 4.18 kernel again, even if you have the 4.20 version installed from somewhere else.)